A porous material made of PTFE fine powder is a noteworthy new material, and some processes for preparing the same have been proposed. Fundamentally, the processes comprise extruding or callendering the PTFE fine powder to form a string, a tube or a film and expanding the formed article before or after sintering.
The conventional processes are roughly classified into the following two groups:
1. Processes comprising expanding the PTFE article at a temperature lower than the melting point of sintered PTFE, namely 327.degree. C., and then sintering the expanded article at a temperature higher than the melting point,
2. Processes comprising expanding the PTFE article at a temperature higher than the melting point of PTFE, preferably higher than the melting point of unsintered PTFE, namely about 345.degree. C. which varies with kinds of the PTFE fine powder and polymerization procedures in emulsion polymerization.
In the latter processes, sintering and expansion of the PTFE article are carried out simultaneously. This reduces the production steps and improves the properties of the porous PTFE material, while it tends to easily break since it is expanded at a temperature higher than the melting point of PTFE.